Self-balancing inverter circuit



Aug. 2s, 1945*. l J E, CRAWLE'Y 2,383,846

SELF BALANCING INVERTER CIRCUIT Filed April l5, 1944 ATTO R N EY v Patented (Aug. .1945

James Bruce Crawley, W

n, D. C., as-

ashingto signor-cto Radio Corporation of America, a corporation of Delaware Application April 15, 1944, Serial No. 581,268

6 Claims. (Cl. 179-171) My present invention relates generally to a circuit for coupling a source of alternating current, unbalanced relative to a reference potential point, to an output circuit whichis substantially balanced relative to said reference point.

, In the past it has been known to employ a phase inverter circuit for coupling a relatively unbalanced (relative to a reference potential point such as ground) source of low frequency currents toy a utilization circuit which is balanced relative to the reference potential point. Generally speaking, a phase inverter circuit of ythe type to which this invention pertains may be classified as a push-pull circuit. This follows from the fact that the inverter circuit output terminals may be connected to a push-pull ampliiler. In operation of phase inverter circuits it has been found desirable to provide measures to overcome the eil'ect of unbalance currents in the outputs of the inver-ter circuit tubes. Hence, various means have been proposed in providing a. measure of self-balancing of the inverter stage f so as to have the 4output currents of the two tubes employed in the phase inverter stage substantially equal.

T It is an important object of my present in? K vention to provide a simple, economical and substantially effective method of, and circuit for, overcoming phase inverter unbalance.

A further object of my invention is to utilize an auxiliary grid in each tube of aphase inverter stage as the eilective element for reducing unbalance.

A more specific object ,of my invention is to provide a phase inverter tube in association with an audio frequency amplifier tube so as to permit the output circuits of the tubes to feed into a push-pull circuit. together with means for applying in eectively opposite phases tol gain control electrodes of said tubes any unbalance currents appearing 'in said common output circuit.

A still more specific object of my invention is to utilize regenerative feedback of unbalance currents, in a. phase inverter stage, to the suppressor grid oi' one tube, and to utilize degenerative feedback of `unbalance currents to the suppressor grid of the second tube of said stage.

The novel features which I believe Ito be characteristic ofv my invention are set forthwith particularity in the appended claims; the in@ vention itself. however, as to both its organization and method of operation will best be un- ,derstood by reference to the following description, taken in connection with the drawingin which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to 4the drawing, let it be as sumed that the source of alternating currents to be amplified is unbalanced relativeto a ref erence potential point, such as ground. lThe source S may be any single-ended audio -frequency amplifier stage. The stage is schematically'represented, since those skilled in the art of amplifying audible and superaudible frequency currents are fully aware of appromiatecircuits. 4

The tube V1 is used to amplify the currents derlved from source B. Assuming that the source S is one providing audio frequency currents, the

condenser Ci couples the high alternating potential output terminal of the source to the input control electrode I of the amplifier tube. The resistor R1 connects the grid i tof ground to provide a direct current return path for the grid. The cathode of the tube is at a relatively tixed potential, such as ground. The low alternating voltage output terminal of source S is grounded. The anode or output relectrode l of tube V1 is connected-to the +B terminal of the usual direct current supply source (not shown) through a series path consisting of resistors Rz and Ra. The screen grid l of tube Vi is vconnected to lthe +B terminal through resi`stor Ra. The latter is preferably of a magnitude such that the screen grid voltage will be of the desired positive value relative to' the positive potential of anode I. Condenser C4 bypasses the screen grid to ground for alternating currents.

The auxiliary control electrode l is coupled by direct current blocking condenser C: to the Junction of resistors Re and Rx. n The control electrode i is returned to ground by a direct current 4'return resistor Rn. The ampliiler tube Vi is shown, by way of example, as a PentOde WDC tube. Hence, each of electrodes I, l and l is a grid electrode. Moreover, the auxiliary control grid i is,'in the illustrated embodiment of my invention, in the form of the usual suppressor grid.- The anode end of resistor Rs is coupled by direct current blocking condenser C z to Vthe upper end of a resistor whose lower -end is grounded. A n intermediate tap ou the resistor divides it into two sections Ik The relative magnitudes of 'the two resistor sections R1 and Re depend ont-he magnitude of alternating voltage to be applied to the input electrode of the inverter tube Va. The latter is shown as a tube V: is connected in common with the cathode y 2 to ground.

The input control grid 1 of tube Y: is con' nected by lead 'I' to the junction of resistor sections Rv and Re. The anode III is connected through series-arranged resistors Rs and R: to the +B terminal of the direct current supply source. Screen grid 8 is connected in common with screen grid 4 to the resistor R4. Auxiliary control grid 9, in the form and location of the usual suppressor grid, is connected in parallel with auxiliary grid 5 to the condenser Cs. The direct current return Ra connects grid 9 to ground. The phase-inverted alternating voltage at the anode end of resistor R5 is applied to lead II by the direct current blocking condenser Cs.

The potentials at leads I I and4 I I are intended to be balanced with respect to ground. Hence. the leads may readily feed into respective control grids of push-pull amplifier tubes. The application of the amplified alternating voltage, ,de-

' rived from across resistor section Rs, to control grid 1 causes tube V2 to invert the phase of the alternating voltage. The alternating voltage at the anode end of Rs is accordingly of opposite phase or polarity with respect to the alternating voltage at the anode end of resistor Ra. The relatiye magnitudes of the voltages at resistors Rz and Rs are determined in the first instance, asy suming that tubes V1 and V: provide substantial equal amplification, by the position of the tap I2 on resistors Rv-Rs. Ideally the voltage outputs of tubes V1 and V2l should be maintained equal.

It is desirable to have resistors Ra and Rs of equal magnitudes; they act as the respective load resistors for the tubes Vi and Va. Since resistor R: is in series with both resistors Rz and Rs, there should be no alternating current voltage across R; ii the outputs of the tub are equal. This follows from the fact that the plate currents of tubes V1 and V2 should be equal and of opposite phase as to their alternating current components. If the tubes are not balanced for some reason, an unbalance alternating voltage will appear across Re. Unbalance" voltage developed across Ra is fed back to the auxiliary grids I and I through condenser C3. In general, the unbalance voltse will be of such polarity as to cause degeneration'in the tube having the greater output, and regeneration in the tube having the lesser output. dual feedback of opposite senses will tend to equalize any difference in amplitudes of the output of the two tubes.

My invention, therefore, contemplates and provides a self-balancing potential inverter system, wherein preferably auxiliary grids, which may be theausual suppressor grids, o! the tubes are used as 'feedback control elements. It will be observed that the unbalance voltage across Rs is actually applied in like phase to suppressorgrlds I vand l, since they are connected in parallel to condenser Cs. Nevertheless, the aforesaid effectively opposed sense feedbacks is secured due to the following action: The unbalance voltage across R; will have the same phase' as the voltage at the anode of the tube having the instantaneously greater output. Hence, the unbalance voltage will lbe 180 degrees out of phase with the grid voltage of that same tube. Considering the suppressor grid of that same tube as being an auxiliary control grid, the action in that tube (of maximum output) is analogous to the effect which would be obtained if the anode voltage were applied to its input control grid I or 1 through a degenerative (resistance-capacity) network.

Consider, now, the action of the other tube at the same instant. Since the two tubes are inversely related in phase, the anode voltages thereof will be 180 degrees out of phase. Therefore, the unbalance voltage at R.: will be 180 degrees out of phase with the alternating voltage at the anode of the tube having the lesser output. If this unbalance voltage were applied through a resistance-capacity network tothe control grid of the tube of lesser output, it would be in phase with the grid voltage. Hence, applying the voitage across Ra to the suppressor grid of the tube of lesser output is equivalent to providing re generative feedback to that tube.

The location of tap il'on resistor Ra-Re is determined either mathematically or by use oi test equipment. By calculating the gain of tube V1 the output voltage of the latter may be determined for any given voltage input, With that value known the tap I2 is located at a point such that the voltage input to the grid 1 of tube Vs is substantially the same as that to the grid I of tube Vi. This may be measured by means of an oscilloscope, vacuum tube meter, or it may be calculated. Y

It will be now seen that resistor Rs, in a manner of speaking, is able to diiferentiate between the tubes so tar as sense of feedback is con cerned because of the ,simple fact thatv the tube having the greater instantaneous output is degenerated whereas regeneration of the signal is obtained in the other tube. The tube which is regenerated tends to increase the gain thereof, whereas the gain of the degenerated tube tends to decrease. There is provided, therefore, a substantially effective, simple and economical method of reducing unbalance in the entire system.

While I have indicated and described a systein for carrying my invention into eifect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular circuit organization shown and described, but that many modiiications may be made without departing from the scope o! my invention as set forthiin the appended claims.

What I claim is:

l. The combination with a signal ampliner tube having input and output electrodes, a phase inverter tube provided with input and output electrodes, means adapted to apply ampliiied signal voltage at said ampliner output electrodes upon the inverter tube input electrodes, separate out put connections Vfrom the respective ampliiler and inverter tube output electrodes;v of the im- .provement which consists of a resistive impedance common to the space current paths of both tubes adapted to develop voltage in response to the output of one tube exceeding the output ci the second tube, and separate parallel connections from said impedance to each tube adapted to apply said last voltage in degenerative phase to said one tube and in regenerative phase to said second tube.

2. The combination with a signal smplier tube having input and output electrodes and an auxiliary control element, a phase inverter tube also provided with input and output electrodes and an auxiliary control element, means adapted to apply amplified signal' voltages at said amplitier output electrodes upon the inverter tube input' electrodes. vente output connections .2,ses,sse

balanced relative to ground from the respective amplifier and inverter tube output electrodes; of

the improvement which consists of 'a resistive one tube and in regenerative phase to said secf` ond tube.

3. The combination with a signal ampliner tube having input and output electrodes, a phase inverter tube provided with input and'output electrodes, means adapted to apply amplified signal voltage at said ampliiier output electrodes upon the inverter tube input electrodes, separate output connections from the respective ampliner and inverter tube output electrodes; oi' the improvement which consists of a resistive impedance common to the space current paths of both tubes adapted to develop voltage in response to the output of one tube exceeding the output o! the'second tube, and separate parallel connections from said impedance to each rtube adapted to apply said last voltage in degenerative phase to said one tube and in regenerative phase to said second tube, each of said tubes including a suppressor electrode, and each o! the feedback connections ybeing made to a respective suppressor electrode.

4. In combination in a system ot the type having a pentode ampliner tube having an un i balanced signal' input connection to its signal grid, a resistive load connected to the tube plate,

'a second tubccf'tnc pcnwactypc navman cathode connected to the ampliner cathode.

3 means forapplying a predetermined magnitude of amplified signal voltage output ot the'am- -pliner'to the second tube signal grid. a resistive load connected to second tube plate, a separate output voltage connection to each respective resistive load; the improvement which comprises g a common lsignal voltage feedback path from the suppressor grid ot each tube to a point common to said two resistive loads.

5. In combination, a signal amplifier tube having input and output electrodes. a second ampliiler tube provided with input and output electrodes. means adapted to apply amplied signal voltage at said nrst' amplifier output electrodes upon the second .tube inputelectrodes, separate output connections :from the respective amplifier tube output electrodes, a resistive impedance common to the space current paths of both tubes adapted to develop unbalance voltage, parallel connections from-said impedance toeach tube adapted to apply said unbalance voltage in degenerative phase to one' tube and in regenerative phase to the second tube.

6. In combination, an ampliiler tube having4 an unbalanced slg'sl input connection to its sunal grid, a resistive loadconnected to the tube plate, a second tube having its cathode connected t0 the amplifier cathode, means for applying a predetermined magnitude of ampliiied signal voltage output of the ampliiier to the second tube signal grid, a resistive load connected to second tube plate, a separate output voltage connection to each respective resistiveload, and a common signal voltage feedback path from a conv'trolgridofeachtubetoapoint commontosaid two resistive loads.

vJAMES BRUCE CRAWLEY, 

